Cellular Biology of Teratoma.

Teratomas are bizarre tumours which contain multiple tissues of kinds foreign to the part of the body in which they arise. Within teratomas these tissues can be organised into recognisable organs, limbs and, in some instances, structures which resemble a complete fetus (1,2,3). These 'monstrous' features give these tumours their name (teras=monster, in Greek). A common example of this bizarre differentiation is the production of hair and skin within benign ovarian teratomas (Fig. 1). The nature of teratomas is due to the fact that they arise from pluripotent cells, i.e. early enbryonic cells or primordial germ cells. Thus a teratoma can contain tissues which are derived from all three embryonic cell layers, and this constitutes the strictest definition of a


INTRODUCTION
Teratomas are bizarre tumours which contain multiple tissues of kinds foreign to the part of the body in which they arise. Within teratomas these tissues can be organised into recognisable organs, limbs and, in some instances, structures which resemble a complete fetus (1,2,3). These 'monstrous' features give these tumours their name (teras=monster, in Greek). A common example of this bizarre differentiation is the production of hair and skin within benign ovarian teratomas (Fig. 1).
The nature of teratomas is due to the fact that they arise from pluripotent cells, i.e. early enbryonic cells or primordial germ cells. Thus a teratoma can contain tissues which are derived from all three embryonic cell layers, and this constitutes the strictest definition of a teratoma (1). If the pluripotent stem cells within a teratoma all undergo differentation to fully mature tissues, then the result is a benign tumour. Persistence of the stem cells within the teratoma results in a malignant tumour. Malignancy may also occur as a consequence of malig-nant transformation occurring in the differentiated tissues within a teratoma. MOUSE TERATOMAS AND TERATOMA CELL LINES Certain strains of mice are susceptible to develops teratomas, and teratomas can also be induced e perimentally, by implanting tissue from fetal gonads ?r early embryos, into suitable sites on a host animal (4/5J-The malignant teratomas of mice are normally referre to as teratocarcinomas, and the pluripotent stem cell 0 these tumours has been named the embryonal carcin0 ma (EC) cell. EC cells have been cultured from teratocar' cinomas, and there are now many EC cell lines availab'e' which possess several intriguing properties: (1) They demonstrate the ability to differentiate in ce culture, e.g. the cell line F9, when treated with retino|C acid, undergoes differentiation to two types of exfa embryonic tissue; parietal and visceral endoderm (6K (2) some lines can be induced, in vitro, to form struc tures which resemble early mouse embryos, known aS embryoid bodies (7); (3) when injected into syngeneic hosts, the EC ce"s produce malignant teratocarcinomas (6,7) and; (4) if EC cells are injected into early mouse embry0 and these are subsequently implanted into pseudopre^ nant host mothers, then chimeric mice are eventual Y born. In these chimeras, the EC cells have actually drf rentiated to form normal (non-malignant) tissues, ',ej they can participate in normal embryonic develops6'1 (7).
The ability of EC cells to form embryoid bodies, andt0 participate in normal embryogenesis during the produ^ tion of chimeric mice, suggests that EC cells are close y related to early embryonic cells. It has, in fact, bee suggested that EC cells are normal embryonic eels' which display malignant potential when placed in 9 inappropriate environment (5,7). This close relations'1'^ between EC cells and early embryonic cells has recent* been further strengthened, by the finding that cell ''ne can be developed from normal mouse embryos. The5 cell lines, known as EK cells, behave essentially identic9 ly to EC cells (8).
It is therefore apparent that, given the right conditio^ the stem cells from mouse teratocarcinomas can eity form malignant tumours, or alternatively different'9 ^ , into normal cells. Many studies have now been earn? out, which aim to discover the molecular events VN/'1lC..
control the differentiation of mouse EC cells. In Par^%C lar, it has been demonstrated thatthedifferentiationof , cells is accompanied by changes in the expression proto-oncogenes (e.g. c-fos (9)) and also of homeo , containing genes (10,11). It    An example of the results of this approach is shown in Fig. 2, which shows cells of four distinctly different morphologies that have been derived from a benign sacrococcygeal teratoma. Immunohistochemical staining with monoclonal antibodies to intermediate filaments is routinely used to characterize such cells, since intermediate filaments are specific markers for different cell types (18). From the benign sacrococcygeal teratoma, we have cultured populations of cells which produce vimentin and keratin, and in primary cultures a small number of cells have been observed which produce either desmin or neurofilament proteins (C. Holmes, personal communication). Thus many diverse cell types have been cultured from a benign sacrococcygeal teratoma, consistent with the extensive somatic differentiation which was observed on standard histological examination. Likewise, we have observed many different cell types in cultures derived from two benign ovarian teratomas.
In benign teratomas, all the stem cells would have differentiated to fully mature tissues, and therefore, it would not be expected that immortal stem cell lines could be derived from such tumours. However, malignant teratomas would be expected to contain such cells. So far, we have been unable to produce any immortal cell lines from the two specimens that we have received from malignant teratomas. Epithelial cells, with the expected morphology for teratoma stem cells, have been observed in cultures derived from a malignant testicular teratoma (Fig. 3), but these cells did not form an established cell line. Few conclusions can be made from the small number of malignant teratomas that we have cultured, except that the production of stem cell lines is obviously not a simple procedure. The failure so far -.

Figure 2
Phase contrast micrographs of four morphologically different cell types, cultured from a benign sacrococcygeal teratoma Figure 2 Phase contrast micrographs of four morphologically different cell types, cultured from a benign sacrococcygeal teratoma could be due either to inappropriate conditions of tissue dispersion or culture, or due to there being very few viable stem cells within the portions of tumour which have been cultured.
In future, we hope to increase the number of samples cultured, by obtaining samples from several other U.K. paediatric oncology centres. In addition, we aim to grow childhood teratomas as xenografts in nude mice, which would then provide a long term source of teratoma tissue, which could be used to optimize tissue culture conditions. The development of permanent cell lines from childhood teratomas will provide the necessary experimental material with which to investigate the cellular and molecular biology of these tumours.  Two colonies of epithelioid cells, growing in a primary culture derived from a malignant testicular teratoma.
(Please contrast micrograph) Figure 3 Two colonies of epithelioid cells, growing in a primary culture derived from a malignant testicular teratoma. (Please contrast micrograph)